1. Field of the Invention
The present invention relates to display presentation of thumbnail images and more particularly to a method for generating a thumbnail image according to a wavelet transformation scheme in the encoding or decoding of an image based on the wavelet transformation scheme.
2. Description of the Related Art
Generally, a thumbnail image refers to a small size image used for image preview and so on.
FIG. 1 is a block diagram illustrating the configuration of a conventional JPEG encoder capable of generating the thumbnail image.
Referring to FIG. 1, a conventional JPEG encoder includes a block divider 100, a Frequency Discrete Cosine Transform (FDCT) 102, a quantization unit 210, a zigzag scanner 106, and an entropy encoding unit 108. The JPEG encoder further includes a discrete cosine transform value extractor 114 connected to the FDCT 102, and an assembling unit 116 in order to generate the thumbnail image.
According to a method by which the JPEG encoder encodes an original image, the block divider 100 divides the original image into blocks of a preset size (e.g. 8 pixel×8 pixel), and the FDCT 102 performs the discrete cosine transformation on each of the divided blocks. The discrete cosine transform value (DCT value) extractor 114 extracts the discrete cosine transform value. The quantization unit 210 receives the extracted DCT values, and performs quantization by using a predetermined quantization matrix. Here, the quantization matrix refers to an integer matrix randomly generated by a designer, etc. The original image is expressed in the form of a matrix with quantization values.
The zigzag scanner 106 receives the original image expressed in the form of a matrix with the quantization values and transforms the original image in the form of integer rows according to a normal zigzag scanning scheme. Then, the transformed original image is input to the entropy encoding unit 108 to be encoded according to an average code length per symbol, determined by an average information amount called ‘entropy’, based on the generation probability of data symbols. The entropy encoding method includes a Huffman coding, an arithmetic coding and the like. In this manner an original image is encoded according to the JPEG encoding scheme.
When the JPEG encoder generates the thumbnail image, the FDCT 102 performs a level shift process for each block to create a predetermined number of coding blocks, and computes the average values of discrete cosine values of the coding blocks. The assembling unit 116 configures an image using these computed average values. For example, in the case where a thumbnail image is generated in the form of a (128×128) pixel coding block including 256 blocks of (8×8) pixels through the level shift process, if the original image has a size of (1024×1024) pixels, the created thumbnail image has a size of (128×128) pixels and corresponds to ⅛th the size of the original image.
Though the JPEG encoding scheme the size of the original image can be reduced without data loss. Another proposed method, referred to as JPEG 2000, is a wavelet transformation method-based encoding scheme capable of improving the compression rate of JPEG encoding scheme by several orders of magnitude. However, the JPEG 2000 encoding scheme is still problematic in that it can not use the JPEG encoding scheme due to the use of the wavelet encoding scheme.
This is because the wavelet transformation applies a low pass filter and a high pass filter to each row of a two-dimensional image, generates four sub-images, (low-low (LL), low-high (LH), high-low (HL) and high-high (HH)), by performing down sampling, and combines the four sub-band images to generate an output image including the same number of samples as compared to the original image.
Accordingly, it is necessary to provide a method for generating the thumbnail image from the original image in the image encoding or decoding scheme such as a JPEG 2000 based on the wavelet transformation scheme.